Studies on the regulation of cell proliferation and tissue size are important to our understanding of developmental biology and tumorigenesis. The long-term goal of the proposed research is to learn the molecular mechanisms that regulate cell proliferation and tissue size during development. We have performed genetic screens in mosaic fruit flies, Drosophila melanogaster, to identify overgrowth mutations. Molecular and genetic characterization of the identified genes have revealed several key regulators of cell proliferation, cell growth, and tissue size, including the lats tumor suppressor. Mutations in lats cause dramatic overproliferation and tumor growth in Drosophila. Two mammalian lats homologs, Latsi and Lats2, have been isolated from humans and mice, and at least one of them also functions as a tumor suppressor. Mice deficient for Latsi develop soft tissue sarcomas and ovarian tumors. Biochemical and genetic experiments have shown that the Lats family proteins are negative regulators of the major cell cycle regulators, CDKs. However, substrates for the Lats kinase are unknown. Several lats-interacting genes have been identified in genetic modifier screens, including Src family members. Interestingly, lats and its interacting genes not only affect cell proliferation, but also deregulate tissue or organ size in Drosophila, a process which is not well understood. The specific aims of this proposal are: (1) Characterizing the roles of Src64 and Src42 in the regulation of cell proliferation and tissue size in imaginal tissues and their relationships with dcsk and lats; and (2) Identifying substrates for the Lats kinase. We hope that defining the relationships among the lats-interacting genes and identifying substrates of the Lats kinase will provide insights into molecular mechanisms regulating cell proliferation and tissue size during development. Since conserved mammalian homologs of Lats and Src are known to be involved in tumorigenesis, it is further hoped that these experiments will also contribute to our understanding of mechanisms underlying human cancer biology.